This invention relates to equipoised articulated supporting arm structures. More particularly, it involves spring counterbalanced arm assemblies for supporting an object in an infinite number of positions.
Equipoised or counterbalanced arm structures have been long utilized to support objects such as lamps. These structures generally employ upper and lower arms each utilizing a parallelogram linkage mechanism. Typically, the two parallel links of the lower arm are pivoted at one end to a swivelling base bracket and at their other ends to a floating bracket. The links of the upper arm are pivoted at one end to the floating bracket and at their other ends to a support bracket for holding the object. The parallelogram linkage allows translations of loads supported at the free end of the linkage without any rotation so that a load such as a lamp or a TV set may be moved to different positions and still retain a constant rotational attitude. A spring coupled to the inner links of each arm provides an opposing moment to counterbalance the moment supplied by the object.
As is taught in U.S. Pat. NO. 2,090,439 to Carwardine, it is known that a mass mounted on the end of a pivoted lever and symmetrically placed can be exactly equipoised by a helical extension spring having a suitable spring rate and arranged to act between a point on the center line of the pivoted lever and a point vertically above its pivot. In other words, for the upper arm, the point of attachment of the spring must be on a vertical line through the point of attachment of the lower link to the floating bracket in order for the object to be exactly counterbalanced throughout its entire arc of movement. This general principle has been utilized in springing the upper arm for a variety of equipoised arm structures as shown, for example, in U.S. Pat. Nos. 3,426,190 to Bobrick and 4,080,530 to Krogsrud.
Special problems are encountered with the upper arm since the springs are in their most relaxed position when the arm is in the extreme outstretched position parallel to the horizon. Of course, in this position a maximum lifting force is needed in order to counterbalance the weight of the object since its mass exerts maximum moments on its suspension at that position. This lifting force is governed by substantially two factors. The first being the strength of the spring and the second being the angle between the longitudinal axis of the spring and the longitudinal axis of the arm, or the components of the spring vector that opposes the gravity force on the supported mass. This latter factor is limited by the shape of the floating bracket. In order to provide an aesthetically pleasing structure, the size of the bracket must be relatively small and, accordingly, the spacing of the spring end from the longitudinal axis of the arm is limited. Moreover, this distance is also limited if it is desired for the springs to be housed within the confines of the links of the arm member, which links can be conveniently formed into nesting U-shaped channels. However, given the current state of the art of spring materials, it is not possible to provide a spring which will counterbalance heavy loads, such as a 25 pound television set supported at the end of a 30 inch parallelogram arm if the width of the arm, and thus of the end brackets, is to be limited to a reasonable distance, such as two inches.
In one attempt to solve this problem, the point of attachment of the spring on the floating bracket was moved beyond the vertical extension of the lower link pivot point. (See U.S. Pat. No. 3,774,873 to Krogsrud). However, the equipoised arm assembly disclosed therein had to rely upon exposed springs to provide the counterbalancing forces. Unfortunately, this does not provide a pleasing appearance which is necessary for commercial acceptance. Even more importantly, by offsetting the point of spring attachment, non-linear counterbalancing forces are supplied by the springs as the object travels through its arc of movement. Accordingly, the object could not be stably positioned at an infinite number of positions which, not only is highly desirable, but may be necessary depending upon the use to which the object is put. For example, if the object is a television set which is being watched by an immobilized bedridden patient, the television set may have to be positioned only at a certain angle for the patient to watch the television.